The present invention relates to a method and apparatus for creating images having "half-tone" or "gray-scale" areas with ink jet printing apparatus.
In thermal ink jet printing, droplets of ink are selectively emitted from a plurality of drop ejectors in a printhead, in accordance with digital instructions, to create a desired image on a copy sheet (e.g. a sheet of plain paper). The printhead typically comprises a linear array of ejectors for conveying the ink to the sheet. The printhead may move back and forth relative to the sheet in a typewriter fashion, for example to print characters, or the linear array may extend across the entire width of a sheet moving relative to the printhead. The ejectors typically comprise capillary channels, or other ink passageways, which are connected to one or more common ink supply manifolds. Ink from the manifold is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated and vaporized by a heating element disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle to the copy sheet. One patent showing the general configuration of a typical ink jet printhead is, for example, U.S. Pat. No. 4,774,530, assigned to the assignee of the present application.
In general, the thermal ink-jet printing process is an all or nothing binary process. In most commercially-available techniques of ink-jet printing, a spot of ink is deposited on the sheet wherever printing is desired, and no ink is deposited where the "white areas" of an image is desired. This binary process is acceptable and in fact desirable when alphanumeric characters or other symbols are being printed. A problem arises, however, when images employing gradations in continuous tones, known as "half tones" or "gray-scales," are to be printed, such as in the printing of photographs. In conventional offset printing, this problem is commonly solved by transforming the continuous tone of the original image into half tone or pseudo-half tone images. Half tone images typically comprise a large number of ink spots of various sizes. The size of the ink spots correspond to the shades or tones to be reproduced. When the spots in the corresponding spaces on the sheet between the spots are small compared to the visual acuity of the human eye, they become subliminal to the eye and not recognized. The spots and the spaces on the sheets thus fuse visually and trick the eye into believing that various shades of continuous tones are seen. Pseudo-half tone images refers to the process of reproducing the continuous tone images or gray scale with a printing device having a fixed printed spot size, and fixed spot-to-spot spacing. The level of gray to be reproduced, thus, is represented by a number of spots which are printed out of a specified line segment or matrix array of printing positions. If the individual spots are sufficiently small, they effect a merger insofar as the eye is concerned to form a visual merger with the spaces between the spots to cause the eye to believe it is seeing various shades of continuous tones.
In ink-jet printing, however, it is difficult to create the variety of spot sizes caused by individual droplets of ink deposited on the sheet to create half tones with reliable precision or accuracy. Various external factors, such as ambient temperature, have a significant effect on the actual sizes of spots created by individual droplets in the ink-jet printing process. Although various techniques have been proposed to ensure the consistency of spot size regardless of temperature, direct control of individual spot size for purposes of half toning is at this time either technically or commercially impractical.
U.S. Pat. No. 4,032,978 discloses a pseudo half tone print generator in which gray scale levels are converted to pseudo half tone print. The gray scale level of the desired image is compared to a reference count for each print position in a print line to generate a print signal upon the level being greater than the count. N adjacent print lines are grouped together to form a series of N.times.M matrices. For each gradation of gray scale, an additional spot is printed within each N.times.M matrix. The arrangement of additional spots within the matrix is such as to minimize the difference between the sums of the count element values for any two adjacent print lines.
U.S. Pat. No. 4,312,005 discloses an ink-jet printing apparatus in which droplets are ejected from a nozzle in the form of a stream and then electrostatically charged by an electrode disposed next to the path of the droplets. A voltage is applied to a first electrode when it is desired to print a spot to deflect ink particles onto a moving sheet, while an alternating voltage is applied to a second deflection electrode to deflect the droplets perpendicular to the deflection direction of the first electrode. Thus, a cluster of droplets are formed on the sheet, each droplet having a diameter equal to a desired spot size.
U.S. Pat. No. 4,394,662 discloses a control system for a "dot printer" wherein the amplitude of a gray scale input signal is modulated prior to application to the recording head, so that the modulated gray scale signal oscillates above and below a threshold level when the input signal is at a low voltage level. The average value of the optical densities thus obtained over a plurality of picture elements equals the average value of the gray scale input which occurs during the period corresponding to the printing of particular picture elements.
U.S. Pat. No. 4,642,653 discloses a half tone system for an ink-jet printer wherein adjacent spots deposited on the sheets are caused to overlap by predetermined amounts, so that the areas of overlap between spots will become visually redundant. That is, to adjacent spots with a large overlap will place less black area on a sheet then two adjacent spots with a small overlap. The pitch for determining the amount of overlap is variable to accomplish gradation in numerous densities.
U.S. Pat. No. 4,680,596 discloses a technique for controlling a group of ink-jet printing heads to selectively supply spots of one color and substantially thick size in a random pattern for decorative purposes. A group of ink-jet heads apply colored spots to the spot positions of a pixel area in accordance with a randomly selected area.
U.S. Pat. No. 4,680,645 discloses a method of creating gray scales with variable spot sizes. A look-up table is used to determine a spot size corresponding to a density value or gray level in the desired image.
U.S. Pat. No. 4,849,768 discloses an apparatus for printing random patterns with fluid jets. A piezoelectric crystal artificially stimulates a flui supply chamber of a fluid jet with coherent acoustic energy to purposely generate standing waves therein. Although sized droplets will be formed at substantially the same frequency from each fluid jet, individual droplets will be formed so as to be out of phase with adjacent neighbors in accordance with the standing wave pattern. By selecting a very short print time, a wide range of random interference patterns can be created on the sheet, particularly patterns simulating natural wood grains including knot holes.